1. Field of the Invention
The present invention relates to gas turbine engines that employ light hydrocarbons as fuel, and more particularly, to an apparatus and method for preventing premature combustion of light hydrocarbons introduced into the gas turbine engines.
2. Description of the Prior Art
A large gas turbine engine generally includes a compressor, a combustion section, and a turbine. An annular flow path for a working fluid extends axially through these sections of the engine. Gas turbine engines produce power through the controlled heating of a compressed working fluid. In an open loop system, the working fluid, typically air, is drawn into the compressor where the air is compressed. The compressed air is then introduced into the combustion section, usually comprised of one or a number of combustion chambers or combustors, where heat is applied to the compressed air. Specifically, the air is mixed with a combustible fuel and burned to produce hot, pressurized gas. The hot combustion gasses are then expanded through the turbine, wherein the hot gasses are directed across blades mounted on a turbine wheel. This flow of hot combustion gasses acts to rotate the turbine wheel. The rotating turbine, which is connected to a gas turbine compressor by means of a drive shaft, provides power for the process compressor. In addition, the turbine may also be used to provide power for other mechanisms, such as generators, pumps or marine propellers. Furthermore, the expulsion of hot gasses through the turbine exhaust nozzle can be used to generate a propulsion force as is typical in jet aircraft engines.
Common construction in combustion chambers includes a metal liner within a cylindrical casing. Air is forced over the surface of the liner, through holes in the liner, and into the interior of the combustion chamber. The combustion chamber is injected with atomized fuel by a nozzle or burner. The initial ignition of the fuel takes place through the firing of a spark plug. Specifically, during startup, the shaft of the gas turbine engine is cranked to starting speed by an external energy source, such as a diesel or electric motor. Once starting speed is attained, fuel and air are introduced into the combustion chamber. A spark plug is then fired to ignite the burner, starting the combustion reaction. Once begun, the combustion reaction is continuous, fed by the continuous injection of fuel into the chamber.
Combustors are now being developed for temperatures of almost 4000.degree. F. The combustor inlet temperature may be anywhere from 250.degree. F. to 2500.degree. F. Due to these temperatures, combustors must be designed to avoid steep temperature gradients within their interiors which would cause warping and cracking of the liners, burners and other metal components within the combustor. Carbon deposits resulting from poorly combusted fuel may also cause local hot spots and distortion of the liner. During start-up, the turbine engine must be brought up to design specifications under highly controlled conditions. For this reason, the start-up and lower power combustion reaction within the combustors is generally carried out under conditions that permit the surfaces within the turbine engine to warm up gradually as the intensity of the combustion reaction is increased.
Generally, combustion in gas turbines is fed by either gaseous or liquid fuels. Turbine engines that are stationary, such as those used in industrial applications, are most commonly fueled by natural gas, if available. In contrast, turbine engines that are mobile, such as those used to propel jet aircraft or drive ships, are more likely to be fueled by liquid hydrocarbons, for example gasoline, diesel fuel, or aviation fuel. In instances when natural gas is not available or practical, stationary gas turbine engines may also use liquid hydrocarbons. Typically, these liquid hydrocarbons are characterized by a specific gravity of at least approximately 0.72. For the purposes of this invention, liquid hydrocarbons having these characteristics are referred to as "heavy hydrocarbons" while liquid hydrocarbons with specific gravities below approximately 0.72 will be referred to as "light hydrocarbons."
Natural gas is desirable as a fuel because of its comparatively low cost relative to the heavy hydrocarbons. However, turbine engines that utilize natural gas must be in locations that have access to a continuous source of natural gas or locations that are fed by natural gas pipelines. Even if a source of natural gas is available, the difficulties of installing and maintaining natural gas pipelines may be prohibitive, especially in remote locations or underdeveloped areas or countries. On the other hand, one drawback to heavy hydrocarbons is their expense and scarcity, especially at times and in locations of high demand such as for automobiles and aircraft.
One solution to the problems associated with heavy hydrocarbons is to employ light hydrocarbons, such as naphtha or butane. Commonly, light hydrocarbons such as these are generated as waste products from refinery processes, and therefore, are readily available at a low cost. These waste products are typically disposed of by flaring into the atmosphere. As gas turbine fuels, these light hydrocarbons are advantageous over heavy hydrocarbons because of their availability and low cost. However, because of their comparatively lower molecular weights and specific gravities, light hydrocarbons typically have lower phase change and flash point temperatures than heavy hydrocarbons. As such, light hydrocarbons are more susceptible to phase changes and more flammable than heavy hydrocarbons. With respect to gas turbine engines, these light hydrocarbons are more likely to prematurely combust within the combustion chamber due to their lower flash points, resulting in temperature gradients that could damage the gas turbine engine, such as, for example, the above-described damage that can occur during start-up procedures and low power conditions. Furthermore, phase changes can create vapor bubbles in the fuel lines that feed the combustor burners, creating interruptions in the continuous fuel flow and potentially causing damage to the turbine engines.
Several prior art methods and devices have been employed to protect the combustor from thermal stresses associated with sudden temperature gradients. For example, it is known in the prior art that the surfaces of the combustor can be coated with a ceramic material to protect against heat stress. Along these same lines, the combustor components can be formed of a high temperature alloys. These solutions, however, do nothing to prevent the light hydrocarbons fuels from prematurely igniting, but only address the problem after the fact. In addition, these solutions are comparatively expensive in relation to standard combustors. Furthermore, in the case of pre-existing gas turbine engines that have been converted to light hydrocarbon fuels, retrofitting turbines with these types of components, if possible, adds both additional expense and would generally require significant downtime.
Another solution to permit use of light hydrocarbons in gas turbine engines is to employ a less flammable fuel during the start-up procedure, i.e., the gas turbine engine is warming up, and under low power conditions. For example, a common practice is to utilize diesel fuel during warm up, and switch over to naphtha once a predetermined load and combustor pressure have been achieved. One drawback to such a configuration is the requirement of dual fuel systems. Specifically, an independent fuel delivery system is required for each type of fuel, even though the fuel delivery systems themselves are generally identical. Another drawback is the excessive use of a more expensive fuel.